In process automation technology, as well as in manufacturing automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are measuring devices, such as, for example, flow measuring devices, fill level measuring devices, pressure and temperature measuring devices, pH-measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, flow, fill level, pressure, temperature, pH-value, and conductivity, respectively. Used for influencing process variables are actuators, such as valves or pumps, via which e.g. the flow of a liquid in a pipeline or the fill level of a medium in a container is changed. The terminology ‘field devices’ as used in connection with the invention includes, thus, all types of measuring devices and actuators. As used in connection with the invention, the terminology ‘field devices’ includes, moreover, all devices, which are applied near to the process and deliver, or process, process relevant information.
In the case of field devices applied in automation technology, electrical current outputs are used to provide signals representing the process variables registered by the field device. An electrical current output provides an interface between the field device and the environment. An electrical current output is unidirectional and enables communication from the field device to a peripheral device representing a load. The peripheral device is, for example, a data logger or a display unit, which is connected with the field device.
In the case of application of the 4-20 mA standard customary in automation technology, an electrical current value between 4 mA and 20 mA represents the process variable within a predetermined range of values of the process variable. For transmitting an alarm state, electrical current values below 4 mA, respectively above 20 mA, are used. In the case of four-conductor field devices, the electrical current outputs can be operated actively, and one speaks in this connection of active electrical current outputs. A four-conductor field device has usually an energy supply input equipped with two input lines for an external energy supply and an electrical current output with two output lines for carrying the measure for the process variable to be determined and/or influenced by the field device.
In the case of known electrical current outputs, the load is connected between a voltage source, which delivers a predetermined voltage, and an electrical current controller, which is connected via a resistor to a reference potential, especially ground. For example, the predetermined voltage of the voltage source amounts to 24 V. The electrical current controller is preferably a transistor, which can be, for example, an FET or a bipolar transistor. The electrical current controller must, as a function of the load connected to the electrical current output and as a function of the electrical current value to be set, convert the excess energy into power loss and therewith into heat. In the case of the known solution, usually a large part of the available energy is converted in the electrical current controller into power loss.
Due to miniaturization of field devices, the draining away of the heat arising from the destruction of the excess energy represents an increasing problem. The associated small housings are only limitedly able to expel the heat to the environment and this leads to an undesired temperature increase of the housings of the field devices. Added to this is the fact that the disposal of energy is viewed as a waste of energy and is increasingly less acceptable to customers.